Periodontitis is a biofilm-mediated disease that afflicts 35% of the adult population in the US, and persistent infections are associated with systemic disease, including cardiovascular disease and stroke. The proliferation of anaerobic bacteria in the subgingival crevice is central to progression of this chronic disease, with Porphyromonas gingivalis being implicated as one of the primary pathogens. We recently reported that a loss in K-antigen capsule synthesis enhanced biofilm formation in P. gingivalis, hence regulation of this virulence factor plays a key role in pathogenicity and biofilm formation. During preliminary studies, we discovered that a gene with high similarity to the DNA binding protein HU (PG0121) is transcribed with the K- antigen capsule synthesis operon and that this histone-like regulatory protein controls expression of the capsule operon. We are now poised to determine the regulatory mechanisms controlling expression of K- antigen capsule. The long-term goal of our research is to elucidate the regulatory mechanisms and signals that control the expression of genes involved in modifying the surface properties of P. gingivalis, and to determine how changes in expression of these genes relate to biofilm development and virulence. The objectives of this application are to characterize the role of HU protein in the synthesis of K-antigen capsule and to investigate the role of the two HU subunits (PG0121 and PG1258) in regulating global gene expression using chromatin immunoprecipitation and microarray analysis. The research proposed in this application is significant because understanding the control of surface property changes is a vital link to understanding the switch this bacterium makes from a quiescent state as a commensal to a virulent pathogen. As an outcome of these studies, we will have established how HU a global regulatory protein controls expression of a key virulence factor, K-antigen capsule. This information will lead to a better understanding of the regulatory networks that either direct P. gingivalis to become a virulent pathogen or to continue to lie low and persist. Our results will potentially lead to the development of new therapeutic strategies for modulating biofilm formation by this oral pathogen.